Bassel Khartabil Safadi, a prominent cyberactivist, was killed two and a half years after his arrest. Wikimedia / Joi Ito

A prominent cyberactivist who was detained on the first anniversary of the 2011 uprising that sparked the Syrian civil war was secretly executed nearly two years ago, his widow said.

Bassel Khartabil Safadi, an open-source software developer who put his skills to use to promote free speech during the uprising, was put to death in October 2015, two and a half years after his arrest, Noura Ghazi Safadi said.

That month, rumours had begun circulating that he had been sentenced to death after being transferred from the regime's notorious Adra prison near Damascus to an unknown location.

His widow gave no indication on her Facebook post late Tuesday how she had confirmed her husband's death.

"Words are difficult to come by while I am about to announce, on behalf of Bassel's family and mine, the confirmation of the death sentence and execution of my husband," she said.

"He was executed just days after he was taken from Adra prison in October 2015. This is the end that suits a hero like him.

"This is a loss for Syria. This is loss for Palestine. This is my loss."

Safadi, who was 34 at the time of his death, was born to a Palestinian father and a Syrian mother. He was well known as an advocate for freedom of information and greater access to the internet.

In 2010, he launched Aiki Lab, which brought together engineers, artists and hackers in Damascus, and also contributed to open-source projects including Creative Commons and Wikipedia.

"Because of Khartabil's work, people gained new tools to express themselves and communicate," British newspaper The Guardian said in a 2015 profile.

Syria had no internet access until 2000, and state censorship and monitoring have remained rampant.

Safadi's expertise was particularly important after the uprising against president Bashar Al Assad broke out in March 2011.

Calls for demonstrations were often issued through Facebook pages, and activists broadcast news and videos through social media.

International human rights groups have long pressed for information on Safadi's fate.

In a 2016 appeal for his release, Human Rights Watch said it believed his detention was "a direct result of his peaceful and legitimate work for the promotion and protection of the right to freedom of expression".

More than 300,000 people have been killed in the civil war that erupted after the uprising, according to the Syrian Observatory for Human Rights.

The Britain-based monitoring group estimates that more than 60,000 of those have been executed or tortured to death in regime prisons.